1. Field of the Invention
The present invention relates to a rotational resistance applying device in a main shaft driving device for a machine tool. More particularly, the present invention is suitable for use in a device that applies rotational resistance to a main shaft rotatably supported by a frame of the machine tool or to a member that rotates together with the main shaft.
2. Description of the Related Art
Hitherto, as a main shaft driving drive used in a machine tool, there is known a rotary index table device that rotates a circular table by rotationally driving a main shaft to which the circular table having a workpiece placed thereon is secured. The rotary index table device is used for indexing an angular position of the circular table by rotationally driving the main shaft to process the workpiece at the indexed angular position. This type of rotary index table device includes a clamping device for holding the circular table at the indexed angular position (that is, the indexed position).
The clamping device is used for setting the main shaft (circular table) in a rotation prevention state at the indexed position. The clamping device is available in three types: a coupling type, a disc type, and a sleeve type. The coupling type stops the rotation of the main shaft by, for example, engagement coupling. The disc type stops the rotation of the main shaft by friction generated by bringing a brake plate (that is, a clamp disc), parallel to the circular table, into contact with an end face of the main shaft and pressing the clamp disc. The sleeve type stops the rotation of the main shaft by friction generated by bringing a clamp sleeve into contact with an outer peripheral face of the main shaft.
As one related art related to a rotary index table device including a clamping device, there exists the technology discussed in Japanese Unexamined Patent Application Publication No. 2006-95668 (Patent Document 1). As a driving unit of a circular table, the rotary index table device discussed in Patent Document 1 uses a direct drive motor (hereunder referred to as “DD motor”) that rotationally drives a main shaft without using a driving transmission unit such as a worm gear. That is, in Patent Document 1, the circular table is set in a rotation prevention state by friction generated by bringing a brake plate into contact with the circular table, secured to the main shaft that is rotationally driven by the DD motor.
Ordinarily, the clamping device is set in a clamp state, and the rotation of the circular table is stopped, to process a workpiece. However, the clamping device may be set in an unclamp state to finish the workpiece while continuously rotating the workpiece. In this case, a main shaft driving device receives a load from a cutter that processes the workpiece. The load varies constantly even under a certain processing condition. Therefore, a rotation state of the circular table resulting from the operation of the main shaft driving device is influenced by the variations in the load received from the cutter.
If a DD motor is used as a driving unit of the main shaft as it is in Patent Document 1, the DD motor is controlled so as to eliminate the influence of the load variations. More specifically, on the basis of the rotational angle (that is, the amount of rotation) of the main shaft that is fed back from a rotation detector of the main shaft driving device, rotational driving of the DD motor is controlled while correcting deviations in the rotational angle of the main shaft caused by the variations in the load received from the cutter. However, the controlling of the DD motor cannot follow variations in the deviations of the rotational angle. In this case, the rotation of the DD motor is not necessarily constant, that is, the DD motor undergoes pulsating rotation.
Even if a driving transmission unit, such as a worm gear, is used as the driving unit of the main shaft, the main shaft may undergo pulsating rotation. Ordinarily, there is backlash in the worm gear. The term “backlash” refers to a gap deliberately provided in a travelling direction in mechanical elements, such as worms (gears), that are fitted to each other and operate. This gap makes it possible for the gears to move freely. However, the backlash causes pulsation to be generated. That is, if the rotational speed at a driving side of the gears engaging each other is changed as a result of the rotational speed being influenced by the variations in the load received from the cutter, the gears at a driven side (main shaft side) swing independently of the driving side within a backlash range due to inertia, thereby generating pulsation in the rotation of the main shaft.
If pulsation is generated in the rotation of the main shaft, pulsation is also generated in the rotation of the circular table secured to the main shaft. As a result, the surface roughness of a finishing surface of the workpiece that is being finished while rotating the circular table is increased. To overcome such a problem, a main shaft driving device that can restrict the generation of pulsation is proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2000-218404 (Patent Document 2)).
The main shaft driving device in Patent Document 2 is related to a lathe that can process holes and grooves in a peripheral face or an end face of the workpiece by a rotating tool mounted to a tool holder while the main shaft is stopped or rotating. The object of Patent Document 2 is to prevent problems in which the shape of the workpiece is deteriorated and in which the precision of the processed surface is reduced from occurring due to a stick-slip phenomenon (pulsation) occurring when the main shaft is rotated at a low speed and the workpiece is processed.
To overcome such problems, the lathe discussed in Patent Document 2 includes a braking device for making uniform the rotation of the main shaft. The braking device includes a brake shoe, a sliding surface member, and a unit for supplying lubricating oil to a sliding contact surface of the sliding surface member. The brake shoe is press-contacted with a rotating member, such as a brake disc or a sleeve, secured to the main shaft. The sliding surface member is mounted to a sliding contact surface of the brake shoe. By rotating at a low speed the main shaft by press-contacting the brake shoe with the rotating member while supplying lubricant oil to the sliding contact surface of the sliding surface member, the difference between a coefficient of static friction and a coefficient of dynamic friction is reduced, so that the stick-slip phenomenon of the main shaft is prevented from occurring.
However, the related art discussed in Patent Document 2 is one in which rotational resistance (braking force) is applied to the main shaft by friction force between the sliding surface member (serving as a pressing member) and the rotating member (serving as a press member that is pressed) by press-contacting the brake shoe, to which the sliding surface member is mounted, with the rotating member at the main shaft. Therefore, in the related art, it is not possible to prevent wear of the rotating member and wear of the sliding surface member that slide while they are pressed. If the rotating member or the sliding surface member wears, the magnitude of rotational resistance changes, thereby making it impossible to provide sufficient rotational resistance. As a result, pulsation is generated in the rotation of the main shaft, thereby increasing the surface roughness of the finishing surface of the workpiece.